Magnum primer dilema

[Overhill]

The wife of a deceased friend gave me 5000 magnum small pistol primers.
The general consensus has been to reduce your powder charge by two grains when using magnum primers
I have been using 5.3 grains of unique with an 115G FMJ for years, as it is the most accurate load for my 9mm pistol. As this is two grains below maximum, i loaded 20 rounds with the magnum primers.
Now, this is where it gets interesting. At the range, I chronographed the 20 rounds with the magnum primers and 20 rounds with the standard primers.
Here are the results.
Magnum average--------1042 fps
Standard average------- 1105 fps
Of course, I have no way of testing pressures but there were no unusual visual signs of excessive pressures.
Now my understanding (correct me if I am wrong) is that magnum primers do not have a bigger bang (for lack of another term) but have a longer spark duration in order to help burn the slow burning powders.
I would like to use these primers in my 40SW and 357SIG also and maybe jump the 9mm up to 5.5 grains to compensate for the lost velocity.
I need some expert opinions.

[Sevens]

I need some expert opinions.

Hmmm... expert or professional? I am niether! They don't pay me to reload and nobody has ever accredited my studies. Anyhow, I'll offer my free OPINION, and I hope it helps!

First is... post questions like this in the Handloading and Reloading forum. But don't worry... a mod will likely move it for you.

As for mag primers, I would not agree that it's a consensus to "drop the powder charge two grains." Think of how much difference two grains is across different calibers. (in fact, I think you meant to say "two-tenths.") If you dropped a .380 load two grains, it wouldn't make it out of the barrel. Likely the same in .38 Special. In any case, no pro or expert would agree to any hard & fast dedicated number as a difference when swapping primers.

The long-standing written rule is to start over well below max and work your way back up when changing ANY primer, whether you move to magnum or simply change brands. If the load you've developed is max or (gulp) over max, then you should start load development over EVEN if you simply change primer lots!

As your pet load seems to be near max, your best bet is to drop is 10% of max (that means start at 4.9gr or 5.0gr) and see how it shoots and increase that load in 0.2gr increments until you get to published max of 5.5gr and see which load works the best.

As to the fact that your chrono is showing similar or hard-to-believe results given that you swapped primers and didn't change the load, I wouldn't be too surprised. First reason could be simply because the primers vary wildly across the commercial brands. Second reason could be that your chrono was giving you different readings because of sun or placement or even the temp you were shooting at compared to the temp of your baseline. Could be that you seated the bullets differently or used different bullets or different brass. Could be that your control group wasn't large enough or your test group isn't large enough and you don't have an accurate average of either group.

In the end, the best move is to stick to the BIG rules if safety is near the top of your list. One of the big rules is to re-develop any loads at or near max when you change a component, especially a component that has a LOT to do with the burn of the charge as does a primer.

[Overhill]

Sevens: A big thanks for your response. Yes i meant to say two tenths. (getting old). The loads were identical except for the primer and were shot under identical conditions. The one thing that i failed to mention is the loads with the magnum primers did not group as well. all testing was done with a Ransom rest.
Thanks again

[rc]

Bullets are getting bumped before the powder charge fully ignites lowering pressures. If the load work well, don't worry about the velocity. You can probably bump up your loads until velocity is the same as factory without issue.

[SSG-Gibb]

I tend to agree with (RC) sounds like the powder isn't burning off all the way with the magnum primers, I had a similar issue with my 45/70. But the safety info is also right on the money and I couldn't have put it better.

[Overhill]

Thank you gentleman. Thats the kind of advise i needed.
Good shooting to all
Conner

[ballardw]

Or find someone that needs the magnums and will trade for regular small pistol.